It is known that the use of multiple antennas at the transmitter and/or the receiver in a wireless communication system can significantly boost the performance of the wireless system. Such antenna configurations have the potential of both improving data rates, improving link reliability and/or increasing coverage.
Precoding is a multi-antenna technique for improving the performance of a multi-antenna system by transforming the information carrying transmit vector so that it better fits the channel conditions. Precoding is usually used in transmitters to mitigate distortion introduced by channel response. There are two common ways of realizing precoding, i.e. codebook based precoding or non-codebook based precoding.
In codebook based precoding, a transmitter and a receiver may have a common precoding codebook, i.e. a finite collection of precoding matrices. The receiver, for example a user equipment, typically decides which matrix or matrices of the codebook are selected for use and feedbacks its index to a transmitter, for example a base station (e.g. an eNB in a Long Term Evolution (LTE) system), on a feedback channel.
In non-codebook precoding, the base station may estimate the uplink channel state, for example based on transmission of uplink sounding reference signals and rely on uplink/downlink channel reciprocity when selecting a precoding matrix to use for the downlink transmission. This is especially of interest for time division duplex (TDD) operation for which the use of the same frequency for uplink and downlink transmission typically leads to a higher degree of downlink/uplink channel reciprocity.
A TDD communication system may have a narrow bandwidth (e.g. a bandwidth of 0.72 MHz) or a wide bandwidth (e.g. a bandwidth of 18 MHz). For a narrow band communication system, it is possible to transmit and detect uplink sounding reference signals over the whole bandwidth and then relatively accurate information about the uplink can be used to determine a precoding matrix for the downlink transmission. However, for a wideband communication system, if uplink sounding reference signals were transmitted and detected over the whole bandwidth, a lot of sounding resources would be consumed and this would increase processing complexity at the base station. To reduce the sounding resource requirement and the base station processing requirement, an approach is to use a small portion of the wide band for uplink sounding reference signal transmission and detection and a precoding matrix determined based thereon for this portion of the band will be used for the whole bandwidth. Another approach as described in WO 2012079202A1 is to adjust the precoding matrix determined for the small portion of the band and use the adjusted precoding matrix for the remaining portions of the band where no uplink sounding reference signal is transmitted.
Currently, when deploying a base station, operators need to manually select one of the approaches for use in a cell served by the base station. That is to say, a precoding mechanism needs to be configured on a cell specific basis. However, even within one cell, air channel is different for different user equipments and neither of the two approaches described above can guarantee to achieve a good performance in all air channel conditions.